Recipe 16.20 Blocking Signals

16.20.1 Problem

You'd like to delay the reception of a signal, possibly to prevent unpredictable behavior from signals that can interrupt your program at any point.

16.20.2 Solution

Use the POSIX module's interface to the sigprocmask(2) syscall. This is available only if your system is POSIX conformant.

To block a signal around an operation:

use POSIX qw(:signal_h);

$sigset = POSIX::SigSet->new(SIGINT);    # define the signals to block
$old_sigset = POSIX::SigSet->new;        # where the old sigmask will be kept

sigprocmask(SIG_BLOCK, $sigset, $old_sigset)
    or die "Could not block SIGINT\n";

To unblock:

defined sigprocmask(SIG_UNBLOCK, $old_sigset)
    or die "Could not unblock SIGINT\n";

16.20.3 Discussion

The POSIX standard introduced sigaction and sigprocmask to give you better control over how signals are delivered. The sigprocmask function controls delayed delivery of signals, and sigaction installs handlers. If available, Perl uses sigaction when you change %SIG.

To use sigprocmask, first build a signal set using POSIX::SigSet->new. This takes a list of signal numbers. The POSIX module exports functions named after the signals, which return their signal numbers.

use POSIX qw(:signal_h);

$sigset = POSIX::SigSet->new( SIGINT, SIGKILL );

Pass the POSIX::SigSet object to sigprocmask with the SIG_BLOCK flag to delay signal delivery, SIG_UNBLOCK to restore delivery of the signals, or SIG_SETMASK to block only signals in the POSIX::SigSet. The most paranoid of programmers block signals for a fork to prevent a signal handler in the child process being called before Perl can update the child's $$ variable, its process id. If the signal handler were called immediately and reported $$ in that handler, it could possibly report its parent's $$, not its own. This issue does not arise often.

16.20.4 See Also

Your system's sigprocmask(2) manpage (if you have one); the documentation for the standard POSIX module in Chapter 32 of Programming Perl